Eliminate Gitaly N+1 queries with notes API

Similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31834,
we see that in https://gitlab.com/gitlab-org/gitlab-ce/issues/65957
there can be hundreds, even thousands, of Gitaly requests in the
`/api/:version/projects/:id/merge_requests/:noteable_id/notes` endpoint.

Previously, the API to retrieve notes generated hundreds of Gitaly calls
to determine whether a system note should be shown to the user. It did
this by:

1. Rendering the Markdown
2. Extracting cross-references from the Markdown
3. Issuing a Gitaly `FindCommit` RPC for every reference to validate
that the commit exists.

The last step is unnecessary because we don't need to display a commit
if the user doesn't have access to the project in the first place.
`RendersNotes#prepare_notes_for_rendering` is already used in
`MergeRequestsController`, which is why we don't see N+1 Gitaly calls
there. We use it here to optimize the note redaction process.

1 files changed, 7 insertions, 0 deletions

diff --git a/spec/requests/api/notes_spec.rb b/spec/requests/api/notes_spec.rb
index 424f0a82e43..6c1e30791d2 100644
--- a/spec/requests/api/notes_spec.rb
+++ b/spec/requests/api/notes_spec.rb
@@ -9,6 +9,13 @@ describe API::Notes do
     project.add_reporter(user)
   end
 
+  context 'when there are cross-reference system notes' do
+    let(:url) { "/projects/#{project.id}/merge_requests/#{merge_request.iid}/notes" }
+    let(:notes_in_response) { json_response }
+
+    it_behaves_like 'with cross-reference system notes'
+  end
+
   context "when noteable is an Issue" do
     let!(:issue) { create(:issue, project: project, author: user) }
     let!(:issue_note) { create(:note, noteable: issue, project: project, author: user) }